Present day commercial processes for treatment of wastewater containing undesired levels of sulfate and heavy metals largely consists of the treatment of water by filtration, ion exchange, reverse osmosis, or electrodialysis. Serious water disposal problems are also encountered in conventional water treatment systems due to the high levels of dissolved minerals and metals present in wastewater, such as industrial and mining wastewaters. The amount of heavy metals and dissolved solids present in water resulting from mining operations, for example, causes particular problems in the disposal of such wastewaters, especially in light of recent Environmental Protection Agency requirements. With the advent of these more stringent water disposal requirements, prior art processes have been found ineffective in efficiently reducing the concentration of heavy metals to acceptable levels.
A significant problem encountered in the treatment of wastewater is the formation of calcium sulfate and calcium carbonate scale. While lime based compounds have been used to soften water, the use of lime provides a source of calcium capable of combining with sulfate and carbonate ions present in wastewater, thus forming insoluble calcium complexes that add to the total suspended solids present in waste water.
Filters used in conventional water treatment processes suffer from clogging problems due to the accumulation of metal and calcium precipitates in the pores of such filters, rendering the filters ineffective and inoperable. To avoid such problems, others have utilized chelating agents and anti-scalants in an attempt to tie up calcium ions, and thereby prevent the formation of undesired calcium compounds such as gypsum and calcium carbonate. Still others have added sulfuric acid in an attempt to reduce the degree of scaling encountered in the treatment of wastewater. Addition of sulfuric acid, however, adds to the level of sulfate ions in wastewater. Moreover, the addition of anti-scalants necessarily increases the cost of water treatment procedures and has not been found to be totally effective in eliminating the scaling problems associated with the formation of undesired calcium compounds.
Many processes for the removal of dissolved heavy metals from industrial wastewater have been proposed. The precipitation of heavy metals by alkaline treatment of wastewater, for example, by treatment with sodium hydroxide, is known in the industry. However, the sludge formed in such processes contains significant amounts of water, creating additional disposal problems due to the volume of the sludge, toxic leaching potential of the sludge, and the significant expense involved in disposing of the sludge. Moreover, in the absence of a high solids content, the sludge resulting from sodium hydroxide/metal precipitation methods precludes the formation of a sufficiently clarified supernatant.
Various methods have been utilized in the past to treat water to remove undesired sulfates, calcium compounds and specific metals, such as magnesium. For example, U.S. Pat. No. 4,059,513 by Zadera discloses a water treatment method for reducing levels of sulfate and hardness by treating process water with calcium hydroxide to form calcium carbonate and calcium sulfate precipitates. After solid calcium sulfate precipitate is removed, raw process water is added to the effluent to allow bicarbonate present in the raw water to react with calcium ions, thereby precipitating calcium carbonate. Sulfuric acid is then added to increase the pH of the solution and thereby prevent scaling.
U.S. Pat. No. 4,462,713 by Zurcher et al. discloses a process in which sodium carbonate and sodium hydroxide are added to saline water to precipitate calcium carbonate. Sodium hydroxide is then added to the resulting filtrate to precipitate magnesium hydroxide. The resulting filtrate is then treated with calcium hydroxide to precipitate calcium sulfate, and, finally, sodium carbonate is added to the resulting filtrate to precipitate excess calcium as calcium carbonate. The final filtrate is then subjected to reverse osmosis, ion exchange or electrodialysis prior to discharge.
U.S. Pat. No. 4,465,597 to Herman et al. discloses an industrial wastewater treatment method in which heavy metals present in wastewater are precipitated to form a slurry and water suitable for environmental discharge is generated. Pursuant to the method, a aqueous mixture of a neutralizing agent absorbed on a carrier is mixed with industrial wastewater containing dissolved heavy metals to precipitate such heavy metals in the form of a slurry which, when allowed to settle, forms a high solids sludge. Herman, et al. discloses the utilization of neutralizing agents, including sodium hydroxide, potassium hydroxide, sodium carbonate, calcium carbonate and lime. Carriers such as particles of sand, silica, alumina, heavy metal oxides or recycled sludge are mixed with the neutralizing agent in a first reactor. The carrier and absorbed neutralizing agent are then mixed with industrial wastewater containing heavy metals in a second reactor to adjust the pH of the wastewater to within a range between about 8.4 and 10.6. At this pH, a substantial portion of the heavy metals are precipitated as oxides/hydroxides. A sludge is formed and water is removed and discharged to the environment. Herman et al., however, do not solve the problems associated with the formation of gypsum in lime-treated wastewater, and specifically Herman et al. fail to address post-precipitation problems encountered in the treatment of wastewater from mining and metal plating operations. Because gypsum formation is a slow, ongoing process, and because it is not effected by changes in the pH of wastewater, the water effluent obtained by Herman, et al. would still suffer from ongoing gypsum formation problems if the treated wastewater had high sulfate concentrations. Such high sulfate containing water is common in mining and metal plating operations. In addition, Herman et al. do not recognize how such gypsum formation can be substantially eliminated by the addition of a material capable of reducing the formation of gypsum, while at the same time facilitating the precipitation of heavy metals contained in wastewater. Herman et al. further do not address the toxic metal characteristic of leaching of sludges produced using their method, nor do Herman et al. discuss the level of total solids (i.e., suspended solids) present in their treated effluent.
None of the prior art methods provide for an economical water treatment method that is effective in the elimination of undesired heavy metals as well as dissolved solids, and that is capable of generating dischargeable water of substantial clarity. Significantly, the elimination of filtering steps and the elimination of costly chemicals such as sodium hydroxide would be desirable in a water treatment process to reduce the cost of such processes and the requisite maintenance involved in the use of filtration systems. A process that is effective in both removing undesired heavy metals from water and in avoiding the problems commonly associated with high concentrations of sulfate and calcium ions, such that water can be treated on-site (i.e., at a mining site operation) and can be subsequently discharged into local water sources, is a long felt but unsolved need. A process for removing heavy metals and suspended solids from water by forming a highly stable, significantly de-watered sludge, would also provide a solution to previously encountered sludge disposal problems.